1. Field of the Invention
The present invention relates to an elastic shaft joint incorporated into a universal joint of, e.g., an automobile steering apparatus, and capable of transmitting a motion of a steering wheel to a steering gear and preventing vibrations of the steering gear from being transmitted to the steering wheel.
1. Related Background Art
An automobile steering apparatus is constructed to transmit to a steering gear a motion of a steering shaft rotated by a steering wheel and to give a steering angle to a front wheel. It is usual that the steering shaft and an input shaft of the steering gear cannot be disposed on a straight line. Therefore, the motion of the steering wheel is transmitted to the steering gear by providing a universal joint between the steering shaft and the input shaft. Further, it has hitherto been a practice to provide the universal joint with a vibration absorbing capability in order to prevent a driver from having an uncomfortable feeling when vibrations transferred from the wheels to the steering gear during traveling of an automobile are further transmitted to the steering wheel. The universal joint is thus given the vibration absorbing capability and therefore incorporates an elastic material such as rubber etc, and this elastic material prevents the vibrations from being transmitted.
Known forms of elastic shaft joint or universal joint incorporating an elastic shaft joint are disclosed in Japanese Patent Laid-Open Publication Nos. 56-39325 (=French Patent Laid-Open No. 2464404), 56-131831, 60-184716 through 184718, 60-215122, 60-215123, 61-201924, Japanese Utility Model Laid-Open Publication Nos. 54-82257, 5-83462, 5-89964, French Patent laid-Open No. 2614985 and U.S. Pat. No. 4,509,775. FIGS. 16-18 show the structure disclosed in Japanese Utility Model Publication No. 5-89964.
A universal joint 1 incorporating this elastic shaft joint includes, as illustrated in FIG. 19, a shaft 2, a first yoke 4 externally fitted via a buffer cylinder 3 to a tip (a left end in FIGS. 19 and 20) of this shaft 2, a second yoke 5, and a joint cross 6 for connecting the second yoke 5 to the first yoke 4. A serration shaft member 7 is, as shown in FIGS. 20 and 21, formed at a portion protruding from one end (a left end in FIG. 20) of the buffer cylinder 3 at the tip of the shaft 2. This serration shaft member 7 is in serration-engagement with a central hole 9 of a transmission piece 8. Accordingly, this transmission piece 8 is fixed to the tip of the shaft 2 and rotates together with this shaft 2. Further, protruded pieces 10, 10, protruding more outwards in a diametrical direction than an outer peripheral surface of the buffer cylinder 3, are integrally formed in two positions opposite to each other in the diametrical direction of the outer peripheral edge of this transmission piece 8.
Among the constructive members of the elastic shaft joint 1, the buffer cylinder 3 is formed in a cylindrical shape, including an elastic material 11 such as rubber, elastomer etc. More specifically, this buffer cylinder 3 has an inner sleeve 12 and an outer sleeve 13 each composed of a metal in a cylindrical shape and disposed concentrically with each other. An outer peripheral surface of the inner sleeve 12 and an inner peripheral surface of the elastic material 11 are joined to each other by baking or bonding, and an inner peripheral surface of the outer sleeve 13 and an outer peripheral surface of the elastic material 11 are similarly joined to each other. The inner sleeve 12 is externally fitted to the tip of the shaft 2, while the outer sleeve 13 is internally fitted to a cylindrical member 14 provided in the first yoke 4, which will hereinafter be explained.
The first yoke 4 includes the cylindrical member 14, and a pair of first arms 15, 15 extending in the axial direction from positions, opposite to each other in a diametrical direction, of one end of this cylindrical member 14 in the axial direction (in the right-and-left direction in FIGS. 19 and 20). The tips of (left ends in FIGS. 19 and 20) of these first arms 15, 15 are formed with first circular holes 16, 16 concentric with each other. Further, notches 17, 17 are formed in positions, off the first arms 15, 15 and opposite to each other in the diametrical direction, of one end of the cylindrical member 14 in the axial direction. A width dimension W of each of the notches 17, 17 is larger than a width dimension w (W&gt;w) of each of the protruded portions 10, 10 of the transmission piece 8. Then, in a state where the shaft 2 is assembled inwardly of the first yoke 4, the respective protruded pieces 10, 10 loosely engage with the notches 17, 17 with slight gaps therebetween.
Moreover, the second yoke 5 has a pair of second arms 18, 18 provided at a spacing away from each other, and is fixedly joined to an end of another shaft 19. Tips of the second arms 18, 18 are formed with second circular holes 20 concentric with each other. The four tips of the joint cross 6 are rotatably supported inwardly of the two pairs of first and second circular holes 16, 20 through bearings such as radial needle bearings etc.
The following is an explanation of the operation of the thus constructed elastic shaft joint 1. When the car travels straight forward, or if a rotational torque applied to the shaft 2 from the steering wheel is small, the protruded pieces 10, 10 of the transmission piece 8 fixed to the tip the shaft 2 assume neutral positions inside the notches 7, 17 formed in the cylindrical member 14 of the first yoke 4 or positions slightly biased from these neutral positions. In each of these states, it never happens that the cylindrical member 14 is brought into direct contact with the transmission piece 8. Further, the small rotational torque described above is transmitted from the shaft 2 via the buffer cylinder 3 to the first yoke 4. In this case, vibrations transmitted to the first yoke 4 from the wheel via the steering gear, the other shaft 19, the second yoke 5 and the joint cross 6 etc, are sufficiently absorbed by the elastic material 11 of the buffer cylinder 3 so as not to be transmitted to the shaft 2.
By contrast, if the rotational torque applied from the steering wheel to the shaft 2 is large, as in the case of giving the front wheel a large steering angle, the protruded pieces 10, 10 impinge on the inside surfaces of the notches 17, 17. As a result, a large proportion of the rotational toque applied to the shaft 2 from the steering wheel is transmitted via the transmission piece 8 to the first yoke 4. In this state, the rotational torque transmitted via the buffer cylinder 3 is limited. Accordingly, even if the rotational torque transmitted via the elastic shaft joint 1 increases, no excessive force acts upon the elastic material 11 of the buffer cylinder 3, and this elastic material 11 is not damaged.
In the case of the thus constructed prior art elastic shaft joint, if the vibrations acting in the axial direction are applied to between the shaft 2 and the first yoke 4, a performance of attenuating these vibrations is poor, and the vibrations acting in the axial direction are easily transmitted to the steering wheel. Specifically, if the vibrations in the axial direction are applied to the first yoke 4, a shearing stress is applied in the axial direction to the elastic material 11 provided between the outer peripheral surface of the inner sleeve 12 and the inner peripheral surface of the outer sleeve 13 of the buffer cylinder 3, with the result that this elastic material 11 is elastically deformed. Since a rigidity of the elastic material 11 against the force in this direction is large, the capability of absorbing such vibrations is poor, and the vibrations in the axial direction are, as stated above, easily transmitted to the shaft 2 on the side of the steering wheel from the first yoke 4.
U.S. Pat. No. 4,509,775 discloses a structure capable of absorbing not only the vibrations in the rotating direction but also those in the axial direction. In the case of the structure discussed in this patent, if a large torque is transmitted, it is required that metallic materials be slid on each other in order to ensure the strength, and it is considered difficult to ensure a sufficient durability. Further, to realize a structure with no backlash, it is also required that a minute gap be controlled, and it is considered that the costs might increase for requiring a dimensional accuracy of the parts.